Radiating coaxial cables, sometimes referred to as "leaky" coaxial cables, have proven to be useful for a wide variety of communications applications such as, for example, transmitting and receiving signals within tunnels or buildings. Generally, radiating cables include a center conductor and an outer conductor separated by a layer of dielectric material, in which the outer conductor is provided with a plurality of slots or apertures along the length of the cable. The slots or apertures serve to couple electromagnetic energy radiating within the cable to fields radiating outside of the cable, such that the cable may be used as a distributed antenna for transmitting or receiving electromagnetic energy. The design of a leaky cable is a delicate balance between the transmission characteristics of the cable and the desired radiation characteristics. The leakage rate and radiation pattern along the cable may be controlled by an appropriate selection of slot size or pattern along the length of the cable. Small changes in the slot pattern or in the design of the cable can significantly change the transmission loss and/or the radiating characteristics of the cable.
There are a variety of known techniques for producing radiating cables having a plurality of slots or apertures on the outer conductor. In one form of cable, the slots or apertures on the outer conductor are formed by a process of milling or punching the surface of an initially solid outer conductor. The manufacture of this form of cable is a relatively slow and expensive process and produces a cable with a stiff, relatively inflexible structure. The bending and unbending, coiling and uncoiling of this form of cable under normal use or manufacture typically deforms the outer conductor and produces unsightly wrinkles on the exterior of the outer conductor which are visible even through the protective outer jacket of the cable. The bending and unbending of the cable may also cause the slots or apertures to become pinched or dilated so as to produce undesired leakage and/or radiation effects along the length of the cable.
Another known form of cable utilizes a braided outer conductor, formed by a braiding machine from groups of wires or "bobbins," each typically including 2 to 10 individual wires. Although cables with braided outer conductors may be coiled and reasonably bent without distortion, the manufacturing of braided leaky cables is a relatively slow and expensive process, requiring the step of periodically stopping the braiding machine to remove some of the wire forming the braid in order to form the apertures.
In view of the aforementioned disadvantages associated with known types of radiating cables, there is a need for an alternative type of radiating cable having a plurality of apertures formed on its outer conductor, which may be used as a distributed antenna for transmitting or receiving electromagnetic energy, but which may be quickly and inexpensively manufactured and which may be coiled and reasonably bent without significant distortion. The present invention is directed to addressing these needs.